This invention relates to an improved method for the in-situ recovery of oil from subterranean hydrocarbon-bearing formations containing low API gravity oil or bitumen. More particularly, the invention relates to an in-situ recovery method wherein improved recovery is realized by optimizing the recovery by the injection of a mixture of an oxygen-containing gas and steam until the recovery efficiency declines, followed by the injection of a mixture of light hydrocarbon and steam, and employing pressurization and drawdown cycles.
The in-situ recovery of low API gravity oil from subterranean hydrocarbon-bearing formations and bitumen from tar sands has generally been difficult. Although some improvement has been realized in the in-situ recovery of heavy oils, i.e., oils having an API gravity in the range of 10.degree. to 25.degree. API, little success has been realized in recovering bitumen from tar sands by in-situ methods. Bitumen can be regarded as a highly viscous oil having an API gravity in the range of about 5.degree. to 10.degree. API and a viscosity in the range of several million centipoise at formation temperature, and contained in an essentially unconsolidated sand, generally referred to as a tar sand.
Extensive deposits of tar sands exist in the Athabasca region of Alberta, Canada. While these deposits are estimated to contain about seven hundred billion barrels of bitumen, recovery therefrom, as indicated above, using conventional in-situ techniques has not been altogether successful. The reasons for the varying degrees of success relate principally to the fact that the bitumen is extremely viscous at the temperature of the formation, with consequent very low mobility. In addition, the tar sand formations have very low permeability, despite the fact they are unconsolidated.
Since it is known that the viscosity of a viscous oil decreases markedly with an increase in temperature, thereby improving its mobility, thermal recovery techniques have been investigated for recovery of bitumen from tar sands. These thermal recovery methods generally include steam injection, hot water injection and in-situ combustion.
Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation. In a conventional throughput steam operation, steam is introduced into the formation through an injection well. Upon entering the formation, the heat transferred by the hot fluid to the formation fluid lowers the viscosity of the oil, thereby improving its mobility, while the flow of the hot fluid serves to drive the oil toward the production well from which it is produced.
Thermal techniques employing steam also utilize a single well technique, known as the "huff and puff" method. In this method, steam is injected via a well in quantities sufficient to heat the subterranean hydrocarbon-bearing formation in the vicinity of the well. Following a period of soak, during which time the well is shut-in, the well is placed on production. After production has declined, the huff and puff technique may again be employed on the same well to again stimulate production. In its application to a field pattern, the huff and puff technique may be phased so that numerous wells are on an injection cycle while others are on a production cycle, which cycles are then reversed.
In the conventional forward in-situ combustion, an oxygen-containing gas, such as air, is introduced into the formation via a well and combustion of in-place crude is initiated adjacent the wellbore. Temperatures of the combustion generally are in the range of 600.degree. to 1200.degree. F. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain a combustion front by burning a portion of the in-place crude or a carbonized deposit resulting from the high temperatures. The injected gas also drives the front through the formation toward a production well. As the combustion front advances through the formation a swept zone consisting ideally of clean sand is created behind the front. Contiguous zones are built up ahead of the front that may include a distillation and cracking zone and a condensation and vaporization zone. The formation of these zones is dependent principally upon the temperature gradients that are created in the formation. As these zones are displaced through the formation, a zone of high oil saturation or an oil bank is established ahead of them, which zone or bank is also displaced toward the production well from which production occurs.
Among the improvements relating to in-situ combustion described in prior art is the injection of water either simultaneously or intermittently with the oxygen-containing gas to scavenge the residual heat, thereby increasing the recovery of oil. Prior art also discloses regulation of the amount of the water injected with the air to improve conformance or sweep efficiency.
Experience has generally shown that in the application of these conventional thermal techniques to the recovery of low API gravity oils and particularly to bitumen recovery from tar sands, conventional thermal techniques have their shortcomings. For example, one difficulty has been that, as the build-up of the oil bank occurs ahead of the thermal front and is displaced through the formation, the bank cools and hence the oil again becomes immobile. The result is that plugging of the formation occurs, thereby making the injection of either the oxygen-containing gas in the case of in-situ combustion, or steam in the case of steam, no longer possible.
An improved thermal method of recovery for low API gravity oil or bitumen from tar sands has been disclosed in U.S. Pat. No. 4,006,778, which utilizes a controlled low-temperature oxidation. According to its teaching, a mixture of an oxygen-containing gas and steam is injected into the formation to generate, and thereafter control, an in-situ low-temperature oxidation. The mixture is injected at a temperature corresponding to the temperature of saturated steam at the pressure of the formation. By this method, the temperature is established and is controlled in the formation at a temperature much lower, i.e., generally in the range of 250.degree. to 500.degree. F., than that of the conventional in-situ combustion process. One of the advantages of the method is the minimization of coking in the formation, which in the conventional in-situ combustion may be excessive and lead to blockage of the formation.
Prior art also teaches the recovery of oil by use of solvents, especially hydrocarbon solvents, either at ambient or elevated temperature. One method is described in U.S. Pat. No. 3,608,638 which employs the injection of a hot hydrocarbon solvent such as toluene or kerosene. The solvent functions principally by dissolving the oil, thereby decreasing viscosity and improving mobility of the fluid. It is also well-known to employ a mixture of hydrocarbon solvent and steam for the recovery of bitumen from tar sand. It is believed that recovery is enhanced by the use of the steam and hydrocarbon mixture because not only is the viscosity of the tar reduced, but also displacement through the sand occurs more rapidly than is possible by the injection of either steam alone or a hydrocarbon solvent. Such a method is described in U.S. Pat. No. 2,862,558 in which a mixture of steam and a normally liquid hydrocarbon is injected into a tar sand formation at a temperature of about 225.degree. to 500.degree. F. and at a pressure of at least 20 psig. More recently, patent literature has described the use of mixtures of depentanized naphtha and steam for recovery of bitumen from tar sand such as described in U.S. Pat. No. 3,945,435 and U.S. Pat. No. 3,946,810. These patents teach that the solvent, having a high aromatic content, is produced from the recovered hydrocarbon and reinjected into the formation with steam at a temperature in the range of 200.degree. to 650.degree. F.
We have now found that, by utilizing a two-step sequence employing the injection of a mixture of an oxygen-containing gas and steam followed by the injection of a mixture of a light hydrocarbon and steam, together with the employment of pressurization and drawdown cycles, enhanced recovery is realized that is higher than that obtained using either the mixture of the oxygen-containing gas and steam or the mixture of the light hydrocarbon and steam alone. Switchover from step (1) to step (2) is made after the recovery efficiency, which is optimized during the first step, begins to show a decline.
Accordingly, it is an object of the present invention to provide an optimized in-situ recovery method for low gravity crudes and bitumen that takes advantage of the beneficial aspects of the use of a mixture of an oxygen-containing gas and steam and a mixture of a light hydrocarbon and steam.